In an electrical communications system, it is sometimes advantageous to transmit information signals (e.g., video, audio, data) over a pair of wires (hereinafter “wire pair” or “differential pair”) rather than a single wire. The signals transmitted on each wire of the wire pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two wires. This transmission technique is generally referred to as “balanced” transmission. When signals are transmitted over wires, electrical noise from external sources such as lightning, automobile spark plugs, radio stations, etc. may be picked up by the wire, degrading the quality of the signal carried by the wire. With balanced transmission techniques, each wire in a wire-pair often picks up approximately the same amount of noise from these external sources. Because approximately an equal amount of noise is added to the signals carried by both wires of the wire pair, the information signal is typically not disturbed, as the information signal is extracted by taking the difference of the signals carried on the two wires of the differential pair, and thus the noise signal is cancelled out by the subtraction process.
Many communications systems include a plurality of differential wire pairs. For example, the typical telephone line includes two differential wire pairs (i.e., a total of four wires), where one wire pair carries the voice signal that travels in one direction (i.e., the voice signal from the calling party to the called party) and the other wire pair carries the voice signal traveling in the opposite direction (i.e., from the called party to the calling party). Similarly, high speed communications systems that are used to connect computers and/or other processing devices to local area networks and/or to external networks such as the Internet typically include four differential wire pairs. In such systems, the wires of the multiple differential pairs are usually bundled together within a cable and thus necessarily extend in the same direction for some distance. Unfortunately, when multiple differential pairs are bunched closely together, another type of noise referred to as “crosstalk” may arise.
“Crosstalk” refers to signal energy from a wire of one differential pair that is picked up by a wire of another differential pair in the communications system. Typically, a variety of techniques are used to reduce crosstalk in communications systems such as, for example, tightly twisting the wires in a cable so that each wire in the cable picks up approximately equal amounts of signal energy from the two wires of each of the other differential pairs included in the cable. If this condition can be maintained, then the crosstalk noise may be significantly reduced, as the wires of each differential pair carry equal magnitude, but opposite phase signals such that the crosstalk added by the two wires of a differential pair onto the other wires in the cable tends to cancel out. While such twisting of the wires and/or various other known techniques may substantially reduce crosstalk in cables, most communications systems include both cables and communications connectors that interconnect the cables and/or connect the cables to computer hardware. Unfortunately, the communications connector configurations that were adopted years ago generally did not maintain the wires of each differential pair a uniform distance from the wires of the other differential pairs in the connector hardware. Moreover, in order to maintain backward compatibility with connector hardware that is already in place in homes and office buildings throughout the world, the connector configurations have, for the most part, not been changed. As a result, many current connector designs generally introduce some amount of crosstalk.
FIG. 1 depicts an exemplary electrical communications system in which crosstalk is likely to occur. As shown in FIG. 1, a computer 1 is connected by a cable 2 that contains a plurality (typically four) wire-pairs to a modular wall jack 5 that is mounted in a wall plate 9. The cable 2 is a patch cord that includes a modular plug 3, 3′ at each end thereof. Modular plug 3 inserts into a modular jack (not pictured in FIG. 1) provided in the back of the computer 1, and modular plug 3′ inserts into an opening 6 in the front side of the modular jack 5, wherein the blades of the plug 3′ mate with respective contacts of the jack 5. In this manner, electrical signals may be communicated from the computer 1 to the modular jack 5. The modular jack 5 includes a connector assembly 7 at the back end thereof that receives and holds wires from a second cable 8 that are individually pressed into slots in the connector assembly 7 to make mechanical and electrical connection. The second cable 8 may connect the computer 1 to, for example, network equipment and/or the Internet.
Pursuant to certain industry standards (e.g., the TIA/EIA-568-B.2-1 standard approved Jun. 20, 2002 by the Telecommunications Industry Association), the communication system of FIG. 1 may include a total of eight wires (four differential pairs). These standards also specify that at the plug-jack mating point the eight wires are aligned in a row, with the four differential pairs specified as depicted in FIG. 2. As shown in FIG. 2, in at least the connection region where the contacts of the modular plug 3′ (see FIG. 1) mate with the contacts of the modular jack 5, the wires of the differential pairs are not equidistant from the wires of the other differential pairs. By way of example, wire 2 (of pair 2) is closer to wire 3 (of pair 3) than is wire 1 (of pair 2) to wire 3. Consequently, differential capacitive and/or inductive couplings occurs between the wires of pairs 2 and 3 that generate near-end crosstalk (NEXT) (i.e., the crosstalk measured at an input location corresponding to a source at the same location) as well as far-end crosstalk (FEXT) (i.e., the crosstalk measured at the output location corresponding to a source at the input location). This crosstalk is an undesirable signal that interferes with the information signal. Similar differential coupling occurs with respect to the other wire pairs in the modular plug 3′ and the modular jack 5.
U.S. Pat. No. 5,997,358 to Adriaenssens et al. (hereinafter “the '358 patent”) describes a two-stage scheme for compensating NEXT for a plug-jack combination. The entire contents of the '358 patent are hereby incorporated herein by reference as if set forth fully herein, as are the contents of U.S. Pat. Nos. 5,915,989; 6,042,427; 6,050,843; and 6,270,381. Connectors described in the '358 patent can reduce the internal NEXT (original crosstalk) between the electrical wire pairs of a modular plug by adding a fabricated or artificial crosstalk, usually in the jack, thereby canceling or reducing the overall crosstalk for the plug-jack combination. The fabricated crosstalk is referred to herein as a compensation crosstalk. One method of reducing NEXT disclosed in the '358 patent is by twice crossing the path of one of the differential pairs within the connector relative to the path of another differential pair within the connector, thereby providing two stages of NEXT compensation. Alternatively, the first and/or second compensation stages can be implemented using discrete components and/or by inducing desired capacitive and/or inductive coupling without actually crossing wire paths. The multi-stage (i.e., two or more) compensation schemes disclosed in the '358 patent can be more efficient at reducing the NEXT than schemes in which the compensation is added at a single stage, especially when the second and subsequent stages of compensation include a time delay that is selected and/or controlled to account for differences in phase between the offending and compensating crosstalk signals. This type of arrangement can include capacitive and/or inductive elements that introduce multi-stage crosstalk compensation, and is typically employed in jack lead frames and printed circuit board structures within jacks. These configurations can allow connectors to meet “Category 6” performance standards set forth in TIA/ETA 568B.2-1 standard, which are primary component standards for mated plugs and jacks for transmission frequencies up to 250 MHz.